Our son has been in day care almost 2 years (since his 2nd birthday). His most recent transition into his newest classroom was the hardest.
He cried every day when we dropped him off and did perfectly well once we were gone. It took about 4 weeks for him to be OK each day (and he stayed in the same facility with several of his friends).
It will take several more weeks - especially since this is such a major adjustment for him. The best thing we found was to continually point out all the great things about his class (how nice the teachers/kids are, what cool things he learned) - it will really help him be excited about going.
We also took some small things into the class to help our son (3.5 years) adjust to the newer kids better. One day it was construction paper so they could make cool crafts, stickers to share, jell-o cups, etc. He was excited to take those things to share with his friends which helped ease the transition.
Good luck! It will get better. He will be your happy little guy again soon.